1. Field of the Invention
The present invention relates to a display apparatus having a cathode ray tube (CRT), and more particularly to a protection circuit and method for protecting a cathode ray tube driving circuit therein against abnormal frequencies of synchronization signals.
2. Background Art
As is well known, a cathode ray tube display apparatus is used as a peripheral apparatus having such a function that a user can see information displayed on its cathode ray tube screen in a personal computer system. It is also known to mute the first grid of the cathode ray tube during a mode conversion or power-on state to prevent a distorted of the raster as contemplated by U.S. Pat. No. 5,463,289 to Moon J. Song and entitled First Grid Muting Circuit.
Additionally, a cathode ray tube display apparatus has a display function which is operated in a text mode or a graphic mode in accordance with the sort of data for display. Thus, when text data is displayed in the cathode ray tube display apparatus, i.e., in a text mode, the computer transfers the text data to the display apparatus, while outputting horizontal and vertical synchronization signals corresponding to the text mode. On the other hand, when there is a necessity of displaying graphic data, the computer transfers the graphic data to the display apparatus, while outputting horizontal and vertical synchronization signal corresponding to the graphic mode to the display apparatus.
I have found that when the frequencies of the synchronization signals H_SYNC and V_SYNC are changed over limited ranges, in the typical contemporary design, a microcomputer will interrupt generation of horizontal and vertical synchronization signals. The microcomputer however, still permits the synchronization signal generator to provide the horizontal and vertical synchronization signals. If the frequencies of these synchronization signals grossly exceeds the limited ranges within a particular interval, the microcomputer will be unable to immediately interrupt the abnormal frequencies. I have noticed that consequently that the microcomputer will not be able to prevent the generation of these abnormal frequencies until expiration of a predetermined period. This may, unfortunately, results in the cathode ray tube driving circuit being damaged by the abnormal frequencies.